(1) Field of the Invention
The present invention relates to a recording medium comprising an amorphous magnetic thin layer consisting mainly of a rare earth metal and an iron family metal used for thermo magnetic recording of a binary code signal.
(2) Description of the Prior Art
A magnetic recording medium, in which a substrate is covered by an amorphous magnetic thin layer consisting of a composite of a rare earth metal and an iron family metal, for example, GdCo, GdFe and the like, has a magnetic anisotropy in the direction perpendicular thereto and exhibits an abrupt change of the coercive force thereof with response to temperature variations near the compensation temperature at which magnetic moments are cancelled by each other between different component materials, so that a magnetic recording medium of this kind is used suitably for high density thermo magnetic recording of a binary code signal.
Where the above mentioned recording medium is used for thermo magnetic recording, numerous benefits can be obtained such as the writing sensitivity of the laser beam is extremely high, and the distinctive Kerr effect is exhibited by the laser beam in the reading-out therefrom, and further the least granular noise is generated on account of the amorphism thereof.
However, on the contrary, a minute magnetic domain formed on the above mentioned recording medium, that is, a recorded bit, which is formed for recording a unit of information, is unstable thermally and magnetically, so that the above mentioned recording medium has a serious defect in that a recorded bit of information is apt to disappear before it is erased intentionally. In other words, it is deficient for high density recording in that to obtain a stably recorded bit of information the diameter of the recorded bit must be larger than a few micrometers.
Furthermore, different structures of the amorphous magnetic thin layer thereof are required respectively for different kinds of the above mentioned materials, namely, GdCo, GdFe and the like. Particularly, it is required to develop a preferably structure of the amorphous magnetic thin layer consisting of GdFe on which a minute stable recorded bit can be obtained, since GdFe has such various industrial merits in comparison with GdCo which is used conventionally. For example, the price of GdFe is lower than about one tenth of that of GdCo and the thin layer consisting of GdFe can be formed easily by conventional evaporation, while that of GdCo can be formed only by sputtering.
On the other hand, to put the above mentioned amorphous magnetic thin layer to practical use as a thermo magnetic recording medium a large area thin layer thereof must be produced. Accordingly, the development of a technique for producing a homogeneous and amorphous magnetic thin layer of large area has been carried out. However, it is well supposed that it is unavoidable in the future also to use practically an amorphous magnetic thin layer having a residual nonhomogeneity to some extent. The most serious difficulty in the practical use of the magnetic thin layer having some nonhomogeneity for the magnetic recording is the unequality of recording sensitivity at different portions of the magnetic thin layer.
For instance, in case the thermo magnetic recording is performed by the irradiation of a laser beam having a constant power, cylindrical magnetic domains, namely, recorded bits having different diameters are formed at different portions thereof. Regarding a rotating recording disc provided with the magnetic thin layer having the nonhomogeneity only in the radial direction, it is possible to obtain recorded bits having the same diameters at different portions thereof by means of presetting of the intensity of the irradiating laser beam appropriately. However, it is more preferable to apply a compensating means for the unequality of recording sensitivity to the magnetic thin layer itself, because various kinds of nonhomogeneity can be coped with by this compensating means.
Furthermore, when an effective compensating means is employed, it is possible that the allowance of nonhomogeneity of the magnetic thin layer can be increased, so that production of the large area magnetic thin layer is technically easier.